Only about half of teen girls in the U.S. have gotten at least one dose, and just a third of teen girls have had all three shots, according to the latest CDC figures.

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HPV is the most common sexually transmitted infection. The vaccine protects against certain types of the human papillomavirus that cause most cases of cervical cancer. The shots work best if given before someone is sexually active so the emphasis has been on giving the shots to 11- and 12-year olds.

The CDC study compared infection rates in girls before and after the vaccine became available. In girls ages 14 to 19, the proportion infected with the targeted strains of HPV fell from about 12 percent to 5 percent, a reduction of 56 percent.

Among girls who had gotten the vaccine, the drop in HPV infections was higher — 88 percent.

There are two vaccines against HPV, but the study mainly reflects the impact of Gardasil, the Merck & Co. vaccine that came on the market in 2006. A second vaccine approved in 2009 — GlaxoSmithKline's Cervarix — probably had relatively little bearing on the results, said the CDC's Dr. Lauri Markowitz, the study's lead author.

Both vaccines are approved for use in males and females — in ages 9 to 26 for females, and 9 to 21 in males. The vaccine was only recommended for boys in late 2011, and the CDC has not yet reported data on how many boys have gotten the shot since then. HPV vaccination requires three shots over 6 months.

An estimated 75 to 80 percent of men and women are infected with HPV during their lifetime. Most don't develop symptoms and clear it on their own. But some infections lead to genital warts, cervical cancer and other cancers. The study didn't look at cervical cancer rates. It can take many years for such cancers to develop, and not enough time has passed to know the vaccine's impact on rates, CDC officials said.

The vaccine's impact was seen even though only 34 percent of the teens in the second group had received any vaccine, and only about 20 percent got all three doses.